The fabrication of features with dimensions of several hundreds of atoms has become quite routine in semiconductor manufacturing. However, conventional fabrication, inspection, and metrology tools are being stretched to their limits.
Atomically precise metrology tool requirements include enhanced resolution, stability and throughput as well as scaled-up array operation and ultra-high vacuum (UHV) compatibility. Another requirement is nano-scale closed-loop position control in all three degrees of freedom.
Scanning probe microscopy covers several related technologies for imaging and measuring surfaces on a fine scale, down to atomic resolution. A scanning probe microscope (SPM) scans an extremely sharp tip of a probe across an object surface while measuring the probe tip-sample interaction via a tunnelling current, atomic force, capacitance, work-function, near field optical detection, or some other means. The imaging signal associated with the scanning probe tip-sample interaction is provided to an imaging system for suitable processing and image rendering.
Conventional SPMs use piezoelectric materials to provide the necessary resolution for scanning. Piezoelectric materials change shape when an electric voltage is applied across them. However, it takes a relatively high voltage—about 100 volts—to make a piezoelectric actuator change shape. This makes for expensive control electronics. In addition, these materials exhibit creep, which compromises the ability to position the probe tip deterministically and with a high degree of stability. With conventional SPMs, the mechanical path between the tip and the sample is sensitive to small temperature variations causing relatively large drift, and is also inherently less mechanically stable than for a miniaturized device. Moreover, it is difficult to implement array architectures using a conventional SPMs.
Fine-scale MEMS (micro-electromechanical systems) positioners provide an alternative to piezoelectric actuators. MEMS-based metrology devices are generally less expensive to manufacture. The driving electronics can be less expensive, since less voltage is required for some types of actuation. However, many MEMS positioners have no electrical signal routing, no position feedback, no integration of sensors inside the actuator, and limited resolution.
Examples of static position sensing in MEMS include:                1) capacitive sensing (charge sensing), where the voltage signal is low and susceptible to noise;        2) piezoresistive sensing, which requires careful thermal balancing and has limitations on ultimate resolution; and        3) optical sensing, which requires tedious alignment and is not amenable to array architectures.        
Dynamic position sensing can make use of lock-in amplifier techniques.
A SPM implemented in a CMOS-MEMS process is described in “A CMOS-MEMS Scanning Probe Microscope with Integrated Position Sensors”, Niladri Sarkar et al. The CMOS-MEMS SPM, with actuators arranged around a central stage that houses a cantilevered probe, allows the integration of all the critical actuation, sensing, and electronic components of an SPM on a wafer that can be batch fabricated in a conventional foundry thereby reducing the size and cost of the SPM while providing the required sensitivity and resolution. However, a main source of disturbance in the position control of the CMOS-MEMS SPM is unwanted parasitic thermal coupling between the actuators and the wafer substrate.
Therefore, there is a need for a MEMS nanopositioner design that provides high resolution and sensitivity while mitigating the problems of thermal coupling.